Cardiac output is a an important parameter in assessing the health or disease state of a subject. For instance, a significantly decreased cardiac output can be predictive of heart failure or the presence of embolisms. Increased cardiac outputs are also found in states of sepsis. Many existing methods that are effective for determining cardiac output have the disadvantage of being invasive. Measurements of cardiac output are commonly made by invasive heart catheterization or by using time-consuming and expensive methodologies such as echocardiography or magnetic resonance imaging. Historically cardiac outputs were determined by the measurement of circulation times. To achieve this dye was injected into the vascular system and the time that the dye took to appear in an unrelated body site was measured.